Manga/Tales of Wedding Rings (or: Lord of the Rings if they tried to fight Sauron with rings of their own)

Yes, the title has an intentionally terrible summary, but the manga does have a lot of shout-outs to LotR. For a more useful summary:

Sato has been in love with his friend Hime for ten years, ever since she and her grandfather appeared before him in a flash of light. When they finally have their first date, Hime informs him that she is leaving for her homeland the next day, and they'll never see each other again. Sato goes to tell her how he feels, but ends up following her through a portal to another world, and finds her in the middle of a wedding to a man she's never met. Then a monster attacks and she kisses Sato instead, making him the Ring King and giving him powers over the five elemental rings to fight the Abyssal Beasts. Now Sato has to find the other four elemental princesses, receive their power as well, and use it to defeat the Abyssal King.

And yes, that means he has to marry them all.

Much better than I expected from a shameless harem series. First off, the series initially plays Cannot Spit It Out straight for Sato and Krystal... until the second chapter, where they both realize they're idiots and just admit they're in love. There are lots of interesting deconstructions/reconstructions of the harem genre and especially Level-Up at Intimacy 5.

Oh, and Marse, the guy who was supposed to be the Ring King, is an aggressive aversion of The Rival. He's basically like "Well, your life is gonna suck now, so I guess I can't complain that you stole the power from me. You want some swordfighting lessons?"

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